1. Field of the Invention
The present invention relates to an image forming method and an image forming apparatus having a high quality gray level image, and in particular, to a technique whereby a transformation characteristic, for transforming the gray level data of an input image into an energy value for printing, is set to a property appropriate for a highlighted portion and a shadow portion.
2. Description of the Related Arts
To record multi-color images, according to one well known method, a gray level transformation characteristic is employed whereby image data for individual colors that are input to an image forming apparatus are divided into blocks that are composed of predetermined areas, and whereby, in accordance with the individual pixel positions in each block, a gray level is transformed into an energy value for image printing, so that the gray level of each pixel is transformed for gray level recording.
According to this image forming method, in the process for transforming the gray level of the image data into energy for a recording head, the transformation characteristic is so set that the maximum energy value applied to the highlighted portion of a printed image is substantially equal to that of the shadow portion.
For this transformation characteristic, however, the quality of the highlighted portion of a printed image may be poor. In order to improve the quality, the energy applied for the transformation characteristic must be increased. When the energy applied is increased, the highlighted portion can be stably printed, but in this case, excessive energy is applied to the shadow portion and causes a harmful influences such as a deterioration of an image quality in that portion.
For example, as the harmful influences, the following points is given. In thin film thermal transfer typed image forming apparatus, image density is lowered. In ink-jet typed image forming apparatus, image confusion or late drying is sometimes accompanied with increase of excess of ink. Further in silver salt typed image forming apparatus, image density is reversed.
To resolve this problem, it is one objective of the present invention to provide an image forming method and an image forming apparatus that increases the printing stability for a highlighted portion, and that can form a high quality image without causing a barrier due to the application of excessive energy to a shadow portion.
To achieve the above objective, according to a first aspect of the present invention, provided is an image forming method that, when gray level image data are input, divides the data into a plurality of blocks and, for gray level recording, converts the gray level image data into multi-valued data, in accordance with a gray level transformation characteristic, determined in accordance with relative pixel positions in each of the blocks, whereby, in accordance with the gray level transformation characteristic, B less than A is established, wherein A denotes a maximum value for multi-valued data when the printing density of the gray level image data is equal to or lower than a half tone, and B denotes a minimum value for an envelope that is obtained by coupling extrema of multi-valued data, relative to transformation characteristic curves for the printing density, that are equal to or higher than the half tone.
For this image forming method, the transformation characteristic is so set that B less than A is established, wherein A denotes a maximum value for multi-valued data when the printing density of the image gray level data is equal to or lower than a half tone, and B denotes a minimum value for an envelope that is obtained by coupling extrema of multi-valued data, relative to transformation characteristic curves for the printing density, that are equal to or higher than the half tone. Therefore, the relative energy applied to the highlighted portion can be increased to improve the stability of the density, and the relative energy applied to the shadow portion can be reduced to remove a barrier due to the excessive application of energy. As a result, a high quality printed image can be obtained.
According to a second aspect of the present invention, wherein said gray level transformation characteristic is so set that B less than 0.85A is established.
For this image forming method, the relative energy applied to the highlighted portion can be increased to improve the stability of the density, or the relative energy applied to the shadow portion can be reduced to remove a barrier due to the excessive application of energy. At least one of the above advantages can be performed certainly. As a result, a high quality printed image can be obtained.
According to a third aspect of the present invention, provided is an image forming apparatus that, when gray level image data are input, divides the data into a plurality of blocks and, for gray level recording, converts the gray level image data into multi-valued data, in accordance with a gray level transformation characteristic, determined in accordance with relative pixel positions in each of the blocks, whereby, in accordance with the gray level transformation characteristic, B less than A is established, wherein A denotes a maximum value for multi-valued data when the printing density of the gray level image data is equal to or lower than a half tone, and B denotes a minimum value for an envelope that is obtained by coupling extrema of multi-valued data, relative to transformation characteristic curves for the printing density, that are equal to or higher than the half tone.
For this image forming apparatus, the transformation characteristic is so set that B less than A is established, wherein A denotes a maximum value for multi-valued data when the printing density of the image gray level data is equal to or lower than a half tone, and B denotes a minimum value for an envelope that is obtained by coupling extrema of multi-valued data, relative to transformation characteristic curves for the printing density, that are equal to or higher than the half tone. Therefore, the relative energy applied to the highlighted portion can be increased to improve the stability of the density, and the relative energy applied to the shadow portion can be reduced to remove a barrier due to the excessive application of energy. As a result, a high quality printed image can be obtained.
According to a fourth aspect of the present invention, wherein said gray level transformation characteristic is so set that B less than 0.85A is established.
For this image forming method, at least one of following advantages can be performed certainly. The relative energy applied to the highlighted portion can be increased to improve the stability of the density, and the relative energy applied to the shadow portion can be reduced to remove a barrier due to the excessive application of energy. As a result, a high quality printed image can be obtained.
According to a fifth aspect of the present invention, a thermal head is employed as a recording head for the image forming apparatus.
Since a thermal head is employed as the recording head of the image forming apparatus, data obtained by a gray level transformation can be output as energy that is to be applied to the thermal head, and the data can be used for high quality printing.
According to a sixth aspect of the present invention, the image forming apparatus employs a heatsensitive transfer material that includes a substantially transparent heatsensitive ink layer, which contains a pigment of 30 to 70 weight % and an amorphous organic polymer of 25 to 60 weight %, which has a softening point of 40xc2x0 C. to 150xc2x0 C. and has a thickness of 0.2 to 1.0 xcexcm, so that the particle diameter of at least 70% of the pigment in the heatsensitive ink layer is equal to or smaller than 1.0 xcexcm, and so that on a white supporting member the optical reflective density of a transferred image is at least 1.0 xcexcm.
Since the heatsensitive transfer material is specified for the image forming apparatus, the obtained color reproductivity and quality are fairly close to those of printed matter, and the quality of the gray level recording that is performed is superior.